events over multiple applications

Hi - I'm new to C# so bear with me. How do I do the equivalent of setting up an event between applications. Basically I have a server app that will trigger events that a client app will need to respond to (and maybe the other way around too) - whats the best way to send a message over the network to get the client app to respond to an event occuring with the server app - also the apps will be doing lots of other stuff so they can't wait for the message, rather respond as and when the message arrives just like events - but I'm told I can't use multithreading (like that had occured to me anyway!). I hope that makes sense. Any help would be greatly appreciated. Sid